Abstract: A double piston pump mechanism includes two pistons operated out-of-phase by respective fluid-actuated cylinders. The pistons suck-in fluid through suction valves and discharge pressurized fluid through pressure valves. The actuation of the cylinders is regulated by two change-over valves which are mechanically connected to respective cylinders. Each change-over valve is actuated when its respective cylinder reaches a fully extended position or a fully retracted position. The change-over valves are connected to a control mechanism which operates to reverse the direction of actuating fluid supplied to the cylinders, the suction valves, and the pressure valves only in response to actuation of both of the change-over valves.